Install npm in Docker explicitly

The Docker builds have started failing because npm is no longer packaged
with node when installing them. This commit adds npm to the list of
packages to install.

Dockerfile

@@ -13,7 +13,7 @@ ENV IN_DOCKER 1
 RUN apt-get update
 RUN apt-get install -y curl nginx
 RUN curl -sL https://deb.nodesource.com/setup_16.x | bash -
-RUN apt-get install -y nodejs
+RUN apt-get install -y nodejs npm
 
 # Copy all our files into the baseimage and cd to that directory
 RUN mkdir /tcd